Last Updated: Thursday March 16 2006 12:49 GMT

What's a water shortage?

This happens when an area does not have enough water to meet the needs of the people who live there.

A shortage is caused by too much demand, or not enough supply.

Too much demand

When humans use more water than we used to, it can cause problems. These are the most common reasons for increased demand:

More people - The world's population is increasing, towns and cities are growing quickly in places where there is not a lot of water.

More farming - In many parts of the world there is not enough rain for crops to be grown without farmers watering, or irrigating them. If farmers start growing more crops they need a lot more water. It takes at least 2,000 litres of water to grow the food needed by one person for one day.

More money - When areas get richer, with more developed economies, the way that people live often changes. They tend to use a lot more water in their homes for things like dishwashers and lawn sprinklers. In developed countries there are also more factories using water. This is not a bad thing in itself, but it can be a problem if there is not a big enough supply of water.

Not enough supply

These are some reasons for an area having less water than it used to:

Not enough rain or drought - This is the most common problem. It happens when there is less rain than would normally be expected. It might be a short problem caused by a change in the weather, or it could be a longer problem caused by changes in an area's climate.

Changes to a river basin - For example if a river is dammed the areas below the dam might get a lot less water. If people living further up the river take a lot of water out, there is sometimes nothing left for people further down.

Wells run dry - In many parts of the world, humans rely on water from an underground source. Underground reservoirs, called aquifers, often take millions of years to fill up. If people remove the water faster than the aquifer can refill, it can cause wells to run dry.